Biaxially oriented polyester film to be laminated onto metal plate and molded

ABSTRACT

A biaxially oriented polyester film to be laminated onto a metal plate and molded, (A) which comprises a copolyester comprising (a) terephthalic acid in an amount of 82 to 100 mol % and 2,6-naphthalenedicarboxylic acid or a combination of 2,6-naphthalenedicarboxylic acid and other dicarboxylic acid in an amount of 0 to 18 mol % of the total of all dicarboxylic acid components and (b) ethylene glycol in an amount of 82 to 100 mol % and cyclohexanedimethanol or a combination of cyclohexanedimethanol and other diol in an amount of 0 to 18 mol % of the total of all diol components, having (c) a glass transition temperature of 78° C. or more and (d) a melting point of 210 to 250° C., and containing (e) porous silica particles with a pore volume of 0.5 to 2.0 ml/g which are agglomerates of primary particles having an average particle diameter of 0.001 to 0.1 μm; and (B) which has the following relationship between the highest peak temperature (Te, °C.) of loss elastic modulus and the glass transition temperature (Tg, °C.): Te−Tg≦30. This film has improved taste-and-flavor retainabilities, particularly taste and flavor retainabilities after a retort treatment, without losing the excellent moldability, heat resistance, impact resistance and retort resistance of a copolyester film.

TECHNICAL FIELD

The present invention relates to a polyester film to belaminated onto ametal plate-and molded. More specifically, it relates to a polyester tobe laminated onto a metal plate and molded, which exhibits excellentmoldability when laminated onto a metal plate to be subjected to a canmaking process such as drawing and which can be used to produce metalcans having excellent heat resistance, retort resistance,taste-and-flavor retainabilities and impact resistance, such as drinkcans and food cans.

BACKGROUND ART

Metal cans are generally coated on interior and exterior surfaces toprevent corrosion. Recently, the development of methods for obtainingcorrosion resistance without using an organic solvent has been promotedfor the purpose of simplifying production process, improving sanitationand preventing pollution. One of the methods is to coat a metal can witha thermoplastic resin film.

That is, studies on a method for making cans, which comprises laminatinga thermoplastic resin film on a plate of a metal such as tin, tin-freesteel or aluminum and drawing the laminated metal plate, are under way.

It gradually becomes clear that a copolyester film is suitable for useas this thermoplastic resin film in terms of moldability, heatresistance, impact resistance and taste-and-flavor retainabilities. Thispolyester film, however, does not always exhibit sufficienttaste-and-flavor retainabilities when a can coated therewith contains adrink whose delicate taste is important, such as green tea, or mineralwater which must be tasteless and odorless, and changes in flavor andtaste of the contents are detected.

JP-A 6-116376 proposes a polyester film to be laminated onto a metalplate and molded, which is made from a copolyester containingalkali-metal elements and a germanium element in specific amounts andwhich has improved flavor retainabilities When this film is used to coata can, however, it exhibits excellent taste-and-flavor retainabilitiesas in a cold pack system in which heat is not applied to the can withcontents, whereas it does not always obtain sufficient taste-and-flavorretainabilities as in a retort treatment in which heat is applied to thecan with contents.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a polyester film tobe laminated onto a metal plate and molded, which solves the aboveproblems of the prior art and which has improved taste-and-flavorretainabilities, particularly taste-and-flavor retainabilities after aretort treatment without losing excellent moldability, heat resistance,impact resistance and retort resistance of a copolyester film.

The other objects and advantages of the present invention will becomeapparent from the following description.

According to the present invention, the above objects and advantages ofthe present invention are attained by a biaxially oriented polyesterfilm to be laminated onto a metal plate and molded, (A) which comprisesa copolyester comprising (a) terephthalic acid in an amount of 82 to 100mol % and 2,6-naphthalenedicarboxylic acid or a combination of2,6-naphthalenedicarboxylic acid and other dicarboxylic acid in anamount of 0 to 18 mol % of the total of all dicarboxylic acid componentsand (b) ethylene glycol in an amount of 82 to 100 mol % andcyclohexanedimethanol or a combination of cyclohexanedimethanol andother diol in an amount of 0 to 18 mol % of the total of all diolcomponents, having (c) a glass transition temperature of 78° C. or moreand (d) a melting point of 210 to 250° C., and containing (e) poroussilica particles with a pore volume of 0.5 to 2.0 ml/g which areagglomerates of primary particles having an average particle diameter of0.001 to 0.1 μm; and

(B) which has the following relationship between the highest peaktemperature (Te, °C.) of loss elastic modulus and the glass transitiontemperature (Tg, °C.):

Te−Tg≦30.

DETAILED DESCRIPTION OF THE INVENTION

The copolyester in the present invention comprises terephthalic acid inan amount of 82 to 100 mol % and 2,6-naphthalenedicarboxylic acid or acombination of 2,6-naphthalenedicarboxylic acid and other dicarboxylicacid in an amount of 0 to 18 mol % of the total of all dicarboxylic acidcomponents.

Illustrative examples of the other dicarboxylic acid include aromaticdicarboxylic acids such as isophthalic acid and phthalic acid; aliphaticdicarboxylic acids such as adipic acid, azelaic acid, sebacic acid anddecanedicarboxylic acid; and alicyclic dicarboxylic acids such ascyclohexanedicarboxylic acid. They may be used alone or in combinationof two or more.

The copolyester in the present invention comprises ethylene glycol in anamount of 82 to 100 mol % and cyclohexanedimethanol or a combination ofcyclohexanedimethanol and other diol in an amount of 0 to 18 mol % ofthe total of all diol components.

Illustrative examples of the other diol include aliphatic diols such asdiethylene glycol, propylene glycol, neopentyl glycol, butanediol,pentanediol and hexanediol; alicyclic diols such ascyclohexanedimethanol; aromatic diols such as bisphenol A; andpolyalkylene glycols such as polyethylene glycol and polypropyleneglycol. They may be used alone or in combination of two or more.

The above copolyester may comprise at least one or both of2,6-naphthalenedicarboxylic acid and 1,4-cyclohexanedimethanol as acopolymer component.

It is particularly preferable that all the dicarboxylic acid componentsof the copolyester consist of terephthalic acid and2,6-naphthalenedicarboxylic acid and that all the diol components of thecopolyester consist of ethylene glycol.

The copolyester in the present invention has a glass transitiontemperature (Tg) of 78° C. or more and a melting point of 210 to 250° C.

If Tg is lower than 78° C., heat resistance will deteriorate andtaste-and-flavor retainabilities after a retort treatment will degradewhen the film of the present invention is laminated onto a metal plateand molded into a metal can. To increase the Tg of the copolyester ofthe present invention to 78° C. or higher, 2,6-naphthalenedicarboxylicacid and cyclohexanedimethanol are used as copolymer components.

The glass transition temperature (Tg) of the copolyester is preferablyin the range of 78 to 90° C.

To obtain a Tg of polyester, a 20-mg film sample is placed in a DSCmeasurement pan, molten by heating on a stage at 290° C. for 5 minutesand solidified by quenching the pan on an aluminum foil laid on ice toobtain a glass transition point at a temperature elevation rate of 20°C./min using the 910 DSC of Du Pont Instruments.

When the melting point is lower than 210° C., the heat resistance of thepolymer deteriorates. On the other hand, when the melting point ishigher than 250° C., the crystallinity of the polymer becomes too highwith the result of impaired moldability.

The melting point of the copolyester is preferably in the range of 210to 245° C.

The melting point of copolyethylene terephthalate is measured inaccordance with a method for obtaining a melting peak at a temperatureelevation rate of 20° C./min using the 910 DSC of Du Pont Instruments.The quantity of a sample is 20 mg.

The intrinsic viscosity (orthochlorophenol, 35° C.) of the copolyesteris preferably in the range of 0.52 to 1.50, more preferably 0.57 to1.00, particularly preferably 0.60 to 0.80. When the intrinsic viscosityis lower than 0.52, impact resistance may be insufficientdisadvantageously. On the other hand, when the intrinsic viscosity ishigher than 1.50, moldability may be impaired.

The content of acetaldehyde in the copolyester is preferably 15 ppm orless, more preferably 12 ppm or less, much more preferably 10 ppm orless.

When the content of the acetaldehyde is larger than 15 ppm, thetaste-and-flavor retainabilities of the contents tend to lowerdisadvantageously.

The concentration of the terminal carboxyl groups of the copolyester ispreferably 40 equivalents/10⁶ g or less, more preferably 35equivalents/10⁶ g or less, much more preferably 30 equivalents/10⁶ g.

When the concentration of the terminal carboxyl groups is higher than 40equivalents/10⁶ g, the amount of the acetaldehyde contained in the filmtends to increase as well and the taste-and-flavor retainabilities ofthe contents are apt to lower. Heat resistance and retort resistance arealso liable to lower and excellent properties obtained by the presentinvention are canceled disadvantageously.

The electric resistivity at 290° C. of the molten copolyester ispreferably set to 5×10⁶ to 1×10⁹ Ω·cm to achieve not only excellentflatness by employing an electrostatic impression process when the filmof the present invention is produced but also excellent laminationproperty and moldability when the film is laminated onto a metal plateand molded into a metal can. When the electric resistivity is lower than5×10⁶ Ω·cm, the taste-and-flavor retainabilities after can makingdeteriorate disadvantageously. On the other hand, when the electricresistivity is higher than 1×10⁹ Ω·cm, film productivity lowers andlamination property and moldability deteriorate disadvantageously.

Although the copolyester in the present invention is not limited by aproduction process thereof, preferred processes for producing a desiredcopolyester are one which comprises subjecting terephthalic acid,ethylene glycol and a copolymer component to an esterification reactionand polycondensing the reaction product until a target degree ofpolymerization is achieved, and one which comprises subjecting dimethylterephthalate, ethylene glycol and a copolymer component to an esterinterchange reaction and polycondensing the reaction product until. atarget degree of polymerization is achieved. The copolyester obtained byone of the above processes (melt polymerization) can be changed into apolymer having a higher degree of polymerization by polymerization inasolid phase (solid-phase polymerization) as required.

The copolyester may contain such additives as an antioxidant, heatstabilizer, viscosity modifier, plasticizer, color modifier, lubricant,nucleating agent and ultraviolet absorber as required.

Preferred examples of the catalyst used for the above polycondensationreaction include antimony compounds (Sb compounds), titanium compounds(Ti compounds) and germanium compounds (Ge compounds). Of these,titanium compounds and germanium compounds are more preferred from theviewpoint of the flavor retainabilities of a film. Preferred titaniumcompounds include titanium tetrabutoxide and titanium acetate. Preferredgermanium compounds include (a) amorphous germanium oxide, (b) finecrystalline germanium oxide, (c) a solution of germanium oxide dissolvedin glycol in the presence of an alkali metal, alkaline earth metal orcompound of these and (d) a solution of germanium oxide dissolved inwater. When an antimony compound and a titanium compound are used incombination, taste-and-flavor retainabilities can be improved and costscan be reduced advantageously.

The copolyester must contain porous silica particles which areagglomerated particles. When only globular or amorphous silica particlesare contained as in the prior art, a remarkable effect of improvingtaste-and-flavor retainabilities cannot be obtained.

The average particle diameter of primary particles forming the poroussilica particles must be in the range of 0.001 to 0.1 μm. When theaverage particle diameter of the primary particles is smaller than 0.001μm, very fine particles are produced by cracking in the stage of aslurry and form agglomerates, causing the formation of pin holes withthe result of deterioration in moldability. On the other hand, when theaverage particle diameter of the primary particles is larger than 0.1μm, the porosity of the particles is lost with the result thattaste-and-flavor retainabilities are not improved.

Further, the pore volume of the porous silica particles must be in therange of 0.5 to 2.0 ml/g, preferably 0.6 to 1.8 ml/g. When the porevolume is smaller than 0.5 ml/g, the porosity of the particles is lostwith the result that taste-and-flavor retainabilities are not improved.On the other hand, when the pore volume is larger than 2.0 ml/g,agglomeration readily occurs by cracking, causing the formation of pinholes with the result of deterioration in moldability.

The particle diameter and amount of the porous silica particles may bedetermined according to film winding property, pin hole resistance andtaste-and-flavor retainabilities. The average particle diameter of theporous silica particles is generally in the range of 0.1 to 5 μm,preferably 0.3 to 3 μm, and the amount thereof is generally in the rangeof 0.01 to 1 wt %, preferably 0.02 to 0.5 wt %.

Although the porous silica particles used in the present invention areagglomerated, the polyester film of the present invention preferablycontains coarse agglomerated particles whose size is 50 μm or more at adensity of 10/m² or less, more preferably 5/m² or less, much morepreferably 3/m² or less. When the number of coarse agglomeratedparticles whose size is 50 μm or more is too large, pin holes arereadily formed and moldability is apt to deteriorate.

To reduce the number of coarse agglomerated particles, it is preferableto filter a molten polymer using a non-woven filter, which is formed ofa thin stainless steel wire having a diameter of 15 μm or less and whichhas an average mesh size of 10 to 30 μm, preferably 15 to 25 μm, as afilter for the production of a film. The porous silica particles aregenerally added to a reaction system, preferably as a slurry containedin a glycol, at the time of a reaction for producing a polyester, forexample, at any time during an ester interchange reaction or apolycondensation reaction when an ester interchange method is employedor at any time when a direct polymerization method is employed. It isparticularly preferable that the porous silica particles be added to thereaction system in the initial stage of the polycondensation reaction,for example, before the intrinsic viscosity reaches about 0.3.

A lubricant is preferably added to the copolyester for the purpose ofimproving film winding property. The lubricant may be inorganic ororganic but preferably inorganic. Illustrative examples of the inorganiclubricant include silica, alumina, titanium oxide, calcium carbonate andbarium sulfate, and illustrative examples of the organic lubricantinclude silicone resin particles and crosslinked polystyrene particles.The lubricant is preferably monodisperse inert spherical particles,which have a particle diameter ratio (long diameter/short diameter) of1.0 to 1.2 and which are not substantially agglomerated, particularlyfrom the viewpoint of pin hole resistance. Illustrative examples of sucha lubricant include completely spherical silica, completely sphericalsilicone resin particles, and spherical crosslinked polystyrene.

The average particle diameter of the inert spherical particles ispreferably 2.5 μm or less, more preferably 0.05 to 1.5 μm.

In the present invention, the average particle diameter of the inertspherical particles is particularly preferably smaller than the averageparticle diameter of the above porous silica particles and in the rangeof 0.05 to 0.8 μm.

The content of the inert spherical particles is preferably 0.01 to 1 wt%.

The lubricant is not limited to the above externally added particles andmay be internally deposited particles obtained by depositing part or allof the catalyst used in the production of a polyester in a reactionstep, for example. It is also possible to use the externally addedparticles and the internally deposited particles in combination.

Two different kinds of particles having different average particlediameters may be used in combination as the lubricant or the inertspherical particles.

The biaxially oriented polyester film of the present invention is madefrom the above copolyester having the following relationship between thehighest peak temperature (Te, °C.) of loss elastic modulus and the glasstransition temperature (Tg, °C.).

Te−Tg≦30

When the value of Te−Tg is larger than 30, the molecule orientation andcrystallinity of the film become too high, with the result of greatdeterioration in moldability. The value of Te, which depends on the typeand amount of the copolymer component, is preferably adjustedparticularly by the stretch ratios of biaxial stretching, stretchingtemperature and heat-setting temperature according to film formationconditions.

Te is obtained at a measurement frequency of 10 Hz and a dynamicdisplacement of ±25×10⁻⁴ cm using a dynamic visco-elastometer.

The relationship between the highest peak temperature (Te) of losselastic modulus and the glass transition temperature (Tg) is preferably

15≦Te−Tg≦25.

The refractive index in a thickness direction of the polyester film ofthe present invention is preferably 1.500 to 1.545, more preferably1.505 to 1.530. When this refractive index is too low, moldabilitybecomes unsatisfactory. On the other hand, when the refractive index istoo high, the structure of the polyester film becomes almost amorphous,whereby heat resistance may lower.

The refractive index in a thickness direction of the polyester film ismeasured by a monochromatic NaD ray with a polarizing plate analyzerattached to the eyepiece side of an Abbe's refractometer. The mountsolution is methylene iodide and the measurement temperature is 25° C.

The center line average roughness (Ra) of the polyester film surface ofthe present invention is preferably 35 nm or less from the viewpoints offilm winding property and taste-and-flavor retainabilities. Ra is morepreferably 15 nm or less, particularly preferably 4 to 15 nm.

The center line average roughness (Ra) of the film surface is measuredin accordance with JIS-B0601 and defined as a value (Ra: nm) obtainedfrom the following expression when a portion having measurement length Lis extracted from a film surface roughness curve in its center linedirection, the center line of the extracted portion is taken as an Xaxis and the direction of the longitudinal stretch ratio is taken as anY axis to represent a roughness curve Y=f(x).

Ra=1/L∫ ₀ ^(L) |f(x)|dx

In the present invention, five portions having a reference length of 2.5mm are measured and the mean of four measurement values excluding thelargest value is taken as Ra.

Since the polyester film of the present invention is used especially infood cans and drink cans, it is preferable that the amount of asubstance dissolved out or dispersed from the film be as small aspossible. However, it is substantially impossible to eliminate thesubstance. Therefore, to use the polyester film of the present inventionin food or drink cans, the amount of the film extracted with ionexchange water at 121° C. for 2 hours is preferably 0.5 mg/cm² or less(0.0775 mg/cm² or less), more preferably 0.1 mg/cm² or less (0.0155mg/cm² or less).

To reduce the amount of the extracted film, it is recommended toincrease Tg of the copolyester.

The polyester film of the present invention preferably has a thicknessof 6 to 75 μm, more preferably 8 to 75 μm, particularly preferably 10 to50 μm. When the thickness is smaller than 6 μm, the polyester film iseasily broken at the time of processing. On the other hand, when thethickness is larger than 75 μm, the polyester film has excessivequality, which is uneconomical.

The metal plate to be laminated with the polyester film of the presentinvention, particularly a metal plate for can making is advantageously aplate of tin, tin-free steel, aluminum or the like. The polyester filmcan be laminated on the metal plate by the following methods (1) and(2), for example.

(1) The metal plate is heated to a temperature higher than the meltingpoint of the film, laminated with the film and quenched. This makes thesurface layer portion (thin layer portion) of the film, which is incontact with the metal plate, amorphous, whereby the film is bonded tothe metal plate.

(2) A primer is coated on the film to form an adhesive layer and thefilm is laminated on the metal plate in such a manner that the adhesivelayer comes into contact with the metal plate. Known resin adhesivessuch as epoxy adhesives, epoxy-ester adhesives and alkyd adhesives canbe used to form the adhesive layer.

The following examples are given to further illustrate the presentinvention. Characteristic properties in the examples were measured inaccordance with the following methods.

(1) Intrinsic Viscosity of Polyester

This is measured in orthochlorophenol at 35° C.

(2) Melting Point of Polyester

This is determined by obtaining a melting peak at a temperatureelevation rate of 20° C./min using the 910 DSC of Du Pont Instruments.The amount of a sample is 20 mg.

(3) Amount of Acetaldehyde (ppm)

The amount of acetaldehyde formed when the film is heated at 160° C. for20 minutes is determined by gas chromatography.

(4) Concentration of Terminal Carboxyl Groups (Equivalent/10⁶ g)

This is measured in accordance with an A. Conix method (Makromal. Chem.26, 226 (1958))

(5) Electric Resistivity of Molten Polymer (Ω·cm)

This is measured in accordance with a method specified in British. J.Appl. Phys. (17, 1149-1154(1966)). The sample is molten at 290° C. andapplied with a DC of 1,000 V, and a stabilized measurement value istaken as an electric resistivity of a molten polymer.

(6) Glass Transition Temperature (Tg) of Polyester

A 20-mg film sample is placed in a DSC measurement pan, molten byheating on a stage at 290° C. for 5 minutes and solidified by quenchingthe pan on an aluminum foil laid on ice. The glass-transitiontemperature of polyester is determined by obtaining a glass transitionpoint at a temperature elevation rate of 20° C./min using the 910 DSC ofDu Pont Instruments.

(7) Highest Peak Temperature (Te) of Loss Elastic Modulus of Film

The loss elastic modulus is obtained at a dynamic displacement of±25×10⁻⁴ cm and a measurement frequency of 10 Hz using a dynamicvisco-elastometer and the highest peak temperature at this point istaken.

(8) Particle Diameter of Particles

Silica particles are scattered in such a manner that each of theparticles is not overlapped with another particle, a metal is depositedon the surface by a gold sputtering device to form a film having athickness of 20 to 30 nm, the film is observed through a scanningelectron microscope at a magnification of 10,000 to 30,000×, and theobtained image is processed with the Luzex 500 of Nippon Regulator Co.,Ltd. The average particle diameter of primary particles is obtained fromthe average particle diameter of 100 particles from the processed image.

The median diameter in equivalent sphere diameter distribution measuredby a centrifugal particle size analyzer is taken as the average particlediameter of particles which are agglomerates of primary particles andseparate particles.

The particle diameter of each particle contained in the film is measuredby the following method.

A small piece of a sample film is set on the sample table of a scanningelectron microscope, and the surface of the film is ion-etched using thesputtering device (JFC-1100 ion etching device) of JEOL Ltd. under thefollowing conditions. The conditions are such that the sample is set ina bell jar, the degree of vacuum is raised to about 3 to 10 Torr and ionetching is carried out at a voltage of 0.25 kV and a current of 12.5 mAfor about 10 minutes. Further, the surface of the film is subjected togold sputtering with the same device and the area equivalent diametersof at least 100 particles are obtained with the Luzex 500 of NipponRegulator Co., Ltd. The mean of measurement values is taken as anaverage particle diameter.

(9) Pore Volume

Using the AUTOSORB-1 of Quantachrome Co., Ltd., the pore volume ofpowder is obtained by measuring the amount of nitrogen absorbed at arelative pressure of 0.998 in accordance with a static volumetric methodon the assumption that the pores of the powder are filled with nitrogen.

The pore volume of the particles contained in the film is measured inaccordance with the following method.

An appropriate amount of a film containing a lubricant is sampled, and asolution of chloroform and HFIP (hexafluoroisopropanol) mixed in a ratioof 1:1 is added to the sample in an excessive amount. The sample is leftto stand in the solution for a whole day to dissolve the samplecompletely. The particles are separated by centrifugation or filtration,a polymer component remaining in the lubricant particles is removed bythe above mixed solution, and the particles are diluted with anddispersed in ethanol and filtered with a straight-hole membrane filter(whose mesh is selected according to the particle diameter of thelubricant). After the end of filtration, the surface of the filter iswashed with ethanol and the ethanol solution is filtered. Afterfiltration, the filter is dried to take out particles. This procedurewas repeated, and the pore volume of the collected particles is obtainedby measuring the amount of nitrogen absorbed at a relative pressure of0.998 in accordance with a static volumetric method using the AUTOSORB-1of Quantachrome Co., Ltd. on the assumption that the pores of theparticles are filled with nitrogen.

(10) Center Line Average Roughness (Ra)

This is measured using the tracer-type surface roughness meter(SURFCORDER SE-30C) of Kosaka Laboratory Co., Ltd. under the followingconditions.

tracer radius: 2 μm

measurement pressure: 0.03 g

cut-off value: 0.25 mm

(11) Deep Drawability

A film is laminated on both sides of a 0.25-mm-thick tin-free steelplate heated to a temperature higher than the melting point of apolyester, cooled with water and cut into a 150-mm-diameter disk-likepiece. The disk-like piece is deep drawn using a drawing dice and apunch in four stages to form a 55-mm-diameter container having seamlessside (to be called “can” hereinafter). This can is observed and testedfor the following items and evaluated based on the following criteria.

(a) Deep Drawability-1

∘: The film is processed without an abnormality and neither whitened norbroken.

Δ: The film at the top portion of the can is whitened.

X: Part of the film is broken.

(b) Deep Drawability-2

∘: The film is processed without an abnormality and exhibits a currentvalue of 0.2 mA or less in an anti-corrosion test on the film on theinterior surface of the can. (The current value is measured whenelectrodes are inserted into the can charged with a 1% NaCl aqueoussolution and a voltage of 6V is applied with the can body functioning asan anode. This test is called “ERV test” hereinafter.)

X: The film is not abnormal but exhibits a current value of 0.2 mA ormore in the ERV test. When an energized portion of the film is magnifiedand observed, a pin-hole-like crack, which is started from the coarselubricant of the film, is observed.

(12) Impact Resistance

Well deep drawn cans are filled with water and cooled to 0° C., and 1.0of the cans are dropped onto a vinyl-chloride-tiled floor from a heightof 30 cm for each test. Thereafter, an ERV test is made on the inside ofeach can.

∘: The films of all the 10 cans exhibit a current value of 0.2 mA orless.

Δ: The films of 1 to 5 cans exhibit a current value of more than 0.2 mA.

X: The films of at least 6 cans exhibit a current value of more than 0.2mA, or the film is cracked after dropped.

(13) Resistance to Heat Embrittlement

Well deep drawn cans are heated at 200° C. for 5 minutes and evaluatedfor impact resistance as described in (8).

∘: The films of all the 10 cans exhibit a current value of 0.2 mA orless.

Δ: The films of 1 to 5 cans exhibit a current value of more than 0.2 mA.

X: The films of at least 6 cans exhibit a current value of more than 0.2mA, or the film is cracked after heated at 200° C. for 5 minutes.

(14) Retort Resistance

Well deep drawn cans are filled with water, subjected to a retorttreatment at 120° C. for 1 hour using a steam sterilizer, and kept at50° C. for 30 days. Ten of the cans are dropped onto avinyl-chloride-tiled floor from a height of 50 cm for each test.Thereafter, an ERV test is made on the inside of each can.

∘: The films of all the 10 cans exhibit a current value of 0.2 mA orless.

Δ: The films of 1 to 5 cans exhibit a current value of more than 0.2 mA.

X: The films of at least 6 cans exhibit a current value of more than 0.2mA, or the film is cracked after dropped.

(15) Taste-and-flavor Retainabilities-1

Well deep drawn cans are filled with ion exchange water and kept atnormal temperature (20° C.) for 30 days. A drink test is made by 30panelists using the immersion liquid to be compared with ion exchangewater as reference. The taste-and-flavor retainabilities of the film areevaluated based on the following criteria.

⊚: Three or less out of 30 panelists feel changes in taste and flavor incomparison with the reference liquid.

∘: Four to 6 out of 30 panelists feel changes in taste and flavor incomparison with the reference liquid.

Δ: Seven to 9 out of 30 panelists feel changes in taste and flavor incomparison with the reference liquid.

X: Ten or more out of 30 panelists feel changes in taste and flavor incomparison with the reference liquid.

(16) Taste-and-flavor Retainabilities-2

Well deep drawn cans are filled with ion exchange water, subjected to aretort treatment in a steam sterilizer at 120° C. for 1 hour and kept atnormal temperature (20° C.) for 30 days. A drink test is made by 30panelists using the immersion liquid to be compared with ion exchangewater as reference. The taste-and-flavor retainabilities of the film areevaluated based on the following criteria.

⊚: Three or less out of 30 panelists feel changes in taste and flavor incomparison with the reference liquid.

∘: Four to 6 out of 30 panelists feel changes in taste and flavor incomparison with the reference liquid.

Δ: Seven to 9 out of 30 panelists feel changes in taste and flavor incomparison with the reference liquid.

X: Ten or more out of 30 panelists feel changes in taste and flavor incomparison with the reference liquid.

EXAMPLES 1 TO 5

and

Comparative Examples 1 and 2

Copolyethylene terephthalates (having an intrinsic viscosity of 0.64 andcontaining 0.1 wt % of porous silica particles which are agglomerates ofprimary particles having an average particle diameter of 0.05 μm andwhich have a pore volume of 1.3 ml/g and an average particle diameter of0.6 μm) prepared by copolymerizing the components shown in Table 1 weredried, melt-extruded and quenched to be solidified so as to obtainunstretched films. The unstretched films were stretched in alongitudinal direction at the stretch ratios and temperatures shown inTable 1 and then in a transverse direction at the stretch ratios andtemperatures shown in Table 1, and further heat-set at 180° C. to obtainbiaxially oriented polyester films.

Each of the obtained films had a thickness of 25 μm and a center lineaverage roughness (Ra) of 0.010 μm and contained no coarse agglomeratedparticles, which are 50 μm or more in size, per m². The glass transitiontemperatures (Tg), the highest peak temperatures of loss elastic moduli(Te), the refractive indices in a film thickness direction and thequantities of extracts with ion exchange water of the films are shown inTable 2 and the evaluation results are shown in Table 3.

TABLE 1 copolymerization melting longitudinal stretching transversestretching copolymer ratio point stretch temperature stretch temperaturecomponent mol % ° C. ratio ° C. ratio ° C. C. Ex. 1 NDC 20 208 3.6 1153.7 115 Ex. 1 ″ 18 213 3.5 110 3.5 115 Ex. 2 ″ 12 228 3.2 110 3.4 120Ex. 3 ″ 6 242 2.9 125 3.1 135 C. Ex. 2 ″ 2 252 2.7 125 2.7 130 Ex. 4CHDM 12 229 3.0 110 3.2 125 Ex. 5 IA 6 228 3.0 115 3.2 130 NDC 6 Ex.:Example, C. Ex.: Comparative Example Notes) copolymer component NDC . .. 2,6-naphthalenedicarboxylic acid CHDM . . . 1,4-cyclohexanedimethanolIA . . . isophthalic acid

TABLE 2 electric resistivity refractive quantity of of molten index inextract with ion Tg Te COOH CH₃CHO polymer thickness Ra exchange water °C. ° C. eq/10⁶ g ppm Ω · cm direction (μm) mg/inch² C. Ex. 1 84 100 30 81.0 × 10⁸ 1.543 0.010 0.21 Ex. 1 83 103 30 8 1.0 × 10⁸ 1.541 0.010 0.16Ex. 2 81 107 33 10 1.0 × 10⁸ 1.518 0.010 0.08 Ex. 3 80 107 33 10 1.0 ×10⁸ 1.514 0.010 0.07 C. Ex. 2 78 106 35 12 1.0 × 10⁸ 1.511 0.010 0.07Ex. 4 79 102 33 10 1.0 × 10⁸ 1.520 0.010 0.19 Ex. 5 78 98 33 10 1.0 ×10⁸ 1.522 0.010 0.21 Notes) COOH content of carboxyl groups CH₃CHOcontent of aldehyde

TABLE 3 deep resistance to taste and flavor drawability impact heatretort retainabilities overall 1 2 resistance embrittleness resistance 12 evaluation C. Ex. 1 ◯ ◯ ◯ ◯ ◯ ⊚ Δ Δ Ex. 1 ◯ ◯ ◯ ◯ ◯ ⊚ ⊚ ⊚ Ex. 2 ◯ ◯ ◯◯ ◯ ⊚ ⊚ ⊚ Ex. 3 ◯ ◯ ◯ ◯ ◯ ⊚ ⊚ ⊚ C. Ex. 2 Δ X — — — — — X Ex. 4 ◯ ◯ ◯ ◯ ◯⊚ ◯ ⊚ Ex. 5 ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ The mark “—” in the table indicates thatevaluation was not made.

As is evident from Table 3, in the case of the present invention(Examples 1 to 5) in which the melting points of copolyesters were 210to 245° C., good results were obtained. When the melting point was lowerthan 210° C. (Comparative Example 1), the obtained film had poor heatresistance and low taste-and-flavor retainabilities after a retorttreatment, while when the melting point was higher than 250° C.(Comparative Example 2), the film was unsatisfactory in terms ofmoldability.

EXAMPLES 6 AND 7

and

Comparative Examples 3 and 4

Copolyethylene terephthalates (having an intrinsic viscosity of 0.62 andcontaining 0.1 wt % of porous silica particles which are agglomerates ofprimary particles having an average particle diameter of 0.03 μm andwhich have a pore volume of 1.0 ml/g and an average particle diameter of0.8 μm) prepared by copolymerizing the components shown in Table 4 weredried, extruded, and quenched to be solidified so as to obtainunstretched films. The unstretched films were stretched and heat-setunder the conditions shown in Table 4 to obtain biaxially orientedpolyester films.

Each of the obtained films had a thickness of 25 μm and a center lineaverage roughness (Ra) of 0.012 μm and contained one coarse agglomeratedparticle, which is 50 μm or more in size, per m². The glass transitiontemperatures (Tg), the highest peak temperatures of loss elastic moduli(Te), the refractive indices in a film thickness direction and thequantities of extracts with ion exchange water of the films are shown inTable 5.

The evaluation results are shown in Table 6. When Tg was 780 C or moreand Te−Tg was 30° C. or less in the present invention (Examples 6 and7), good results were obtained. When Tg was lower than 78° C.(Comparative Example 3), the film had poor heat resistance and lowtaste-and-flavor retainabilities after a retort treatment. When Te−Tgwas higher than 30° C. (Comparative example 4), the film had lowmoldability.

TABLE 4 copolymerization melting longitudinal stretching transversestretching Heat setting copolymer ratio point stretch temperaturestretch temperature temperature component mol % ° C. ratio ° C. ratio °C. ° C. C. Ex. 3 IA 6 235 3.1 115 3.2 125 170 NDC 3 Ex. 6 IA 3 235 3.1115 3.2 125 170 NDC 6 Ex. 7 NDC 9 235 3.1 115 3.2 125 170 C. Ex. 4 NDC 9235 3.2 115 3.3 125 170 Notes) copolymer component IA . . . isophthalicacid NDC . . . 2,6-naphthalenedicarboxylic acid

TABLE 5 refractive quantity of index in extract with ion Tg Te Te-Tgthickness exchange water ° C. ° C. ° C. direction mg/inch² C.Ex.3 77 10225 1.513 0.19 Ex.6 79 105 26 1.511 0.11 Ex.7 81 109 28 1.516 0.08 C.Ex.481 113 32 1.512 0.04

TABLE 6 deep resistance to taste-and-flavor drawability impact heatretort retainabilities overall 1 2 resistance embrittleness resistance 12 evaluation C. Ex. 3 ◯ ◯ ◯ ◯ ◯ ⊚ Δ Δ Ex. 6 ◯ ◯ ◯ ◯ ◯ ⊚ ⊚ ⊚ Ex. 7 ◯ ◯ ◯◯ ◯ ⊚ ⊚ ⊚ C. Ex. 4 ◯ X — — — — — X The mark “—” in the table indicatesthat evaluation was not made.

EXAMPLES 8 TO 11

and

Comparative Examples 5 to 8

The average particle diameter and pore volume of the primary particlesof porous silica were changed as shown in Table 7 in Example 2 to obtainbiaxially oriented polyester films.

The results are shown in Table 8. When the average particle diameter ofthe primary particles of porous silica was 0.01 to 0.1 μm and the porevolume thereof was 0.5 to 2.0 ml/g in the present invention (Examples 8to 11), good results were obtained. When the average particle diameterof the primary particles was larger than 0.1 μm (Comparative Example 6)and the pore volume was smaller than 0.5 ml/g (Comparative Example 7),the porosity of silica lowered and the effect of improvingtaste-and-flavor retainabilities was small. When the average particlediameter of the primary particles was smaller than 0.001 μm (ComparativeExample 5) and the pore volume was larger than 2.0 ml/g (ComparativeExample 8), agglomeration easily occurred, pin holes were formed at thetime of molding, and moldability was unsatisfactory.

TABLE 7 porous silica average particle refractive quantity of diameterof pore index in extract with ion Tg primary particles volume thicknessexchange water ° C. μm ml/g direction mg/inch² C.Ex.5 81 0.0008 1.51.518 0.08 Ex.8 81 0.005 1.5 1.518 0.07 Ex.9 81 0.09 1.5 1.518 0.09C.Ex.6 81 0.11 1.5 1.518 0.10 C.Ex.7 81 0.05 0.4 1.518 0.08 Ex.10 810.05 0.6 1.518 0.10 Ex.11 81 0.05 1.8 1.518 0.07 C.Ex.8 81 0.05 2.11.518 0.10

TABLE 8 deep resistance to taste-to-flavor drawability impact heatretort retainabilities overall 1 2 resistance embrittleness resistance 12 evaluation C. Ex. 5 Δ X — — — — — X Ex. 8 ◯ ◯ ◯ ◯ ◯ ⊚ ⊚ ⊚ Ex. 9 ◯ ◯ ◯◯ ◯ ⊚ ⊚ ⊚ C. Ex. 6 ◯ ◯ ◯ ◯ ◯ ⊚ Δ Δ C. Ex. 7 ◯ ◯ ◯ ◯ ◯ ⊚ Δ Δ Ex. 10 ◯ ◯ ◯◯ ◯ ⊚ ⊚ ⊚ Ex. 11 ◯ ◯ ◯ ◯ ◯ ⊚ ⊚ ⊚ C. Ex. 8 Δ X — — — — — X The mark “—”in the table indicates that evaluation was not made.

EXAMPLES 12 TO 18

Copolyethylene terephthalates (containing 0.1 wt % of porous silicawhich is an agglomerate of primary particles having an average particlediameter of 0.03 μm and which have an average particle diameter of 0.8μm and a pore volume of 1.0 ml/g)(may be abbreviated as co-PEThereinafter) were prepared using the acid components, diethylene glycol,alkali metal compounds, polycondensation catalysts and phosphoruscompounds shown in Table 9. The copolyethylene terephthalates weredried, melt-extruded at 280° C., and quenched to be solidified so as toobtain unstretched films. The unstretched films were stretched to 3.0times in a longitudinal direction at 110° C. and then to 3.0 times in atransverse direction at 120° C., and heat-set at 180° C. to obtain25-μm-thick biaxially oriented films. The films had a center lineaverage roughness (Ra) of 0.012 μm and other characteristic propertiesthereof are shown in Table 9 and Table 10.

TABLE 9 characteristic properties of polymer electric dicarboxylic acidresistivity of components DEG poly- molten (molar ratio) componentcondensation polymer A B (mol %) catalyst (Ω · cm) Ex.12 TA(90) NDC(10)1.5 Sb₂O₃ 1.0 × 10⁸ Ex.13 TA(90) NDC(10) 1.0 ″ 1.0 × 10⁸ Ex.14 TA(90)NDC(10) 3.0 ″ 2.5 × 10⁸ Ex.15 TA(82) NDC(18) 1.5 ″ 1.0 × 10⁸ Ex.16TA(94) NDC(6) ″ ″ 1.0 × 10⁸ Ex.17 TA(90) NDC(10) ″ ″ 1.5 × 10⁸ Ex.18TA(90) NDC(10) ″ Sb(OCOCH₃)₃ 1.0 × 10⁸ characteristic properties ofpolymer average particle diameter of melting terminal carboxyl lubricantintrinsic Tg point group (μm) viscosity (° C.) (° C.) (equivalents/10⁶g) Ex.12 0.5 0.70 81 232 33 Ex.13 ″ ″ 82 231 37 Ex.14 ″ ″ 78 226 30Ex.15 ″ ″ 83 213 33 Ex.16 ″ ″ 80 242 33 Ex.17 ″ ″ 81 232 20 Ex.18 ″ ″ 81232 33 Notes) TA: terephthalic acid NDA: 2,6-naphthalenedicarboxylicacid DEG: diethylene glycol

TABLE 10 amount of contents of metals in film Te Te-Tg acetaldehyde A SbSb + M + P (Sb + M)/P (° C.) (° C.) (ppm) (ppm) (mmol %) (mmol %) (mmol%/mmol %) Ex. 12 102 21 10 10 40 60 4.0 Ex. 13 103 21 12 ″ 50 75 ″ Ex.14 100 22 8 0 30 45 ″ Ex. 15 99 16 10 10 40 60 ″ Ex. 16 109 29 10 ″ ″ ″″ Ex. 17 102 21 3 5 30 45 ″ Ex. 18 102 21 10 10 40 60 ″ Notes) A: totalamount of alkali-metal elements remaining in film Sb: concentration ofSb metal element remaining in film M: concentration of catalyst metalelement other than Sb remaining in film P: concentration of phosphoruselement remaining in film

EXAMPLES 19 TO 25

Copolyethylene terephthalates (intrinsic viscosity of 0.64), prepared bycopolymerizing 10 mol % of 2,6-naphthalenedicarboxylic acid as acopolymer component and containing lubricants A having average particlediameters shown in Table 11 (porous silica which is an agglomerate ofprimary particles having an average particle diameter of 0.03 μm andwhich has a pore volume of 1.0 ml/g) and lubricants B having averageparticle diameters shown in Table 11 (completely spherical silica havinga particle diameter ratio of 1.07 and a relative standard deviation of0.09) in proportions shown in Table 11 were dried, melt-extruded andquenched to be solidified so as to obtain unstretched films. Theunstretched films were stretched to 3.2 times in a longitudinaldirection at 120° C. and then to 3.3 times in a transverse direction at130° C., and heat-set at 180° C. to obtain unstretched films.

Each of the obtained films had a thickness of 25 μm, a glass transitiontemperatures (Tg) of 81° C., the highest peak temperature of losselastic modulus (Te) of 100° C. and a Te−Tg of 19° C. Further, therefractive index in a thickness direction of each of the films was1.520, the quantity of an extract with ion exchange water was 0.12mg/inch², and the center line average roughness (Ra) is shown in Table11.

The evaluation results are shown in Table 12. Cans made by using thefilms of the present invention are satisfactory in terms of resistanceto heat embrittlement, retort resistance and impact resistance and haveimproved taste-and-flavor retainabilities, particularly taste-and-flavorretainabilities after a retort treatment, and excellent deepdrawability.

TABLE 11 lubricant A (porous silica) lubricant B (spherical silica)average particle average particle diameter (μm) content (wt %) diameter(μm) content (wt %) Ra (nm) Ex. 19 0.3 0.5 0.1 0.1 10 Ex. 20 0.6 0.1 0.10.5 12 Ex. 21 0.8 0.05 0.1 0.2 12 Ex. 22 0.8 0.1 0.1 0.2 14 Ex. 23 0.80.1 0.5 0.02 15 Ex. 24 1.5 0.1 0.5 0.1 22 Ex. 25 2.3 0.02 0.5 0.1 31

TABLE 12 deep resistance to taste-to-flavor drawability impact heatretort retainabilities overall 1 2 resistance embrittlement resistance 12 evaluation Ex. 19 ◯ ◯ ◯ ◯ ◯ ⊚ ⊚ ⊚ Ex. 20 ◯ ◯ ◯ ◯ ◯ ⊚ ⊚ ⊚ Ex. 21 ◯ ◯ ◯◯ ◯ ⊚ ⊚ ⊚ Ex. 22 ◯ ◯ ◯ ◯ ◯ ⊚ ⊚ ⊚ Ex. 23 ◯ ◯ ◯ ◯ ◯ ⊚ ⊚ ⊚ Ex. 24 ◯ ◯ ◯ ◯ ◯⊚ ◯ ◯ Ex. 25 ◯ ◯ ◯ ◯ ◯ ⊚ ◯ ◯

The polyester film to be laminated onto a metal plate and molded of thepresent invention has improved taste-and-flavor retainabilities,particularly taste-and-flavor retainabilities after a retort treatment,without losing the excellent heat resistance, impact resistance andretort resistance of copolyester and does not experience a reduction inmoldability, when laminated onto a metal plate and subjected to a canmaking process such as deep drawing for making a metal can. Therefore,it is extremely useful as a film for a metal container.

What is claimed is:
 1. A biaxially oriented polyester film to belaminated onto a metal plate and molded, (A) which comprises acopolyester comprising (a) terephthalic acid in an amount of 82 to 100mol % and 2,6-naphthalenedicarboxylic acid or a combination of2,6-naphthalenedicarboxylic acid and other dicarboxylic acid in anamount of 0 to 18 mol % of the total of all dicarboxylic acid componentsand (b) ethylene glycol in an amount of 82 to 100 mol % andcyclohexanedimethanol or a combination of cyclohexanedimethanol andother diol in an amount of 0 to 18 mol % of the total of all diolcomponents, having (c) a glass transition temperature of 78° C. or moreand (d) a melting point of 210 to 250° C., and containing (e) poroussilica particles with a pore volume of 0.5 to 2.0 ml/g which areagglomerates of primary particles having an average particle diameter of0.001 to 0.1 μm; and (B) which has the following relationship betweenthe highest peak temperature (Te, °C.) of loss elastic modulus and theglass transition temperature (Tg, °C.): Te−Tg≦30, wherein thecopolyester also contains inert spherical particles which have aparticle diameter ratio wherein the (long diameter/short diameter) ratioof 1.0 to 1.2 and an average particle diameter of 2.5 μm or less andwhich are not substantially agglomerated.
 2. The film of claim 1,wherein all the dicarboxylic acid components of the copolyester consistof terephthalic acid and 2,6-naphthalenedicarboxylic acid and all thediol components of the copolyester consist of ethylene glycol.
 3. Thefilm of claim 1, wherein the glass transition temperature (Tg) of thecopolyester is in the range of 78 to 90° C.
 4. The film of claim 1,wherein the melting point of the copolyester is in the range of 210 to245° C.
 5. The film of claim 1, wherein a concentration of terminalcarboxyl groups of the copolyester is 40 eq/10⁶ g or less.
 6. The filmof claim 1, wherein a content of acetaldehyde in the copolyester is 15ppm or less.
 7. The film of claim 1, wherein the electric resistivity ofthe copolymer, as a melt at 290° C., is in the range of 5×10⁶ to 1×10⁹Ω·cm.
 8. The film of claim 1, wherein the average particle diameter ofthe porous silica particle agglomerates is in the range of 0.1 to 5 μm.9. The film of claim 1, wherein the content of the porous silicaparticles is 0.01 to 1 wt %.
 10. The film of claim 1, wherein the inertspherical particles are silica particles.
 11. The film of claim 1,wherein the average particle diameter of the inert spherical particlesis smaller than that of the porous silica particles and is in the rangeof 0.05 to 0.8 μm.
 12. The film of claim 1, wherein the content of theinert spherical particles is in the range of 0.01 to 1 wt %.
 13. Thefilm of claim 1, which has the following relationship between thehighest peak temperature (Te) of loss elastic modulus and the glasstransition temperature (Tg): 15≦Te−Tg≦25.
 14. The film of claim 1, whichhas a refractive index in a film thickness direction of 1.500 to 1.545.15. The film of claim 1, which has a center line average surfaceroughness (Ra) of 35 nm or less.
 16. The film of claim 1, which has acenter line average surface roughness of 15 nm or less.
 17. The film ofclaim 1, wherein the quantity of an extract obtained by an extractiontreatment with ion exchange water at 121° C. for 2 hours is 0.5 mg/inch²(0.0775 mg/cm²) or less.
 18. The film of claim 1, which has a thicknessof 6 to 75 μm.
 19. A process for the production of a laminate comprisingthe step of laminating the film of claim 1 on a metal plate.
 20. Aprocess for making a metal can comprising the step of deep drawing thelaminate of claim 19.